This invention relates to contact heater mounting assemblies which enable resistance heating elements to be adhesively applied to surfaces intended to be heated. More particularly, the invention relates to improvements which facilitate the manufacture and handling of rear window defogger mounting assemblies.
Prior art contact heater mounting units have been rather cumbersome to manufacture and particularly difficult to handle, primarily because of the required protection of adhesive layers having relatively high cohesion levels. Such layers are contained on individual heating elements. For example, one common means of protecting the high level adhesive layer or the so-called "glass mounting" adhesive on each element is a protective cover layer superimposed thereover. The integrity of the high level adhesive layer is extremely critical, because the adhesive must be capable of permanently affixing the heating elements to a mounting surface upon removal of the cover layer. The high level adhesive must have a cohesion which is strong enough to attach the heating element to the glass, yet the cover layer must be capable of being easily removed in the field. For this purpose a release agent such as silicone is generally applied to the cover layer, and thus facilitates its removal.
Physical contact, however, between the cover layer and the high level adhesive creates a major disadvantage in that the release agents employed, such as silicone, tend to migrate into the adhesive and to decrease its cohesion capacity over a period of time. Moreover, a relatively fine balance must be achieved between the cohesive levels of the two adhesives, as the cover layer must separate cleanly from the high level adhesive without prematurely pulling the heating element away from the low level adhesive and hence from a backing or support sheet. Finally, after the heating elements are pressed onto the mounting surface, the low level adhesive must allow release of the backing sheet from the heating elements without pulling the heating elements away from the mounting surface.
A second means of protecting and thus insuring the integrity of the high level adhesive is to provide for a spacing or gap between the surface of the protective cover and the high level adhesive. One means by which this has been accomplished has been the inclusion of undulations in the protective cover layer, whereby portions of the cover layer are spaced from the adhesive on the heating elements. Alternatively, the support contains the undulations, wherein the heating element is recessed into the support layer by amounts sufficient to provide a gap between the cover layer and the high level adhesive. Both of the latter means, however, require special manufacturing and handling techniques, even though they avoid the special problem of migration of release agents. For example, such units cannot be conveniently rolled or coiled, as the layer having the undulations will collapse upon being rolled. The latter will cause the protective layer to come into contact with and to thus damage the high level adhesive layer.